1. Field of the Invention
The invention relates to electric power plants comprising at least one battery adapted to be supplied with electric power from a power source with temporally random availability.
2. Description of the Related Art
Such plants are known which comprise a battery adapted to be recharged with current by means of photovoltaic panels. The energy (power) available in a temporally random manner is then solar radiation. These plants are used in particular in developing countries, in regions having no local or national electricity supply network. Usually, to control the charging and discharging of the battery, the voltage across the terminals of the battery is monitored and a predetermined top voltage threshold and a predetermined bottom voltage threshold are used. Thus, when the voltage across the terminals reaches the bottom threshold, the users are disconnected so as to safeguard the battery from excessive discharging. Once the voltage across the terminals reaches the top threshold, the battery is disconnected from the panels and, after a timeout of a few minutes, the battery is made available for the delivery of current by reestablishing the connection with the users. An advantage of this device is its simplicity.
However, such a method of control has numerous drawbacks. Firstly, in practice there is no strict correlation between the instantaneous voltage across the terminals of the battery and its state of charge. In particular, it is possible for the voltage to be high while the charge of the battery is very low.
Moreover, the battery conventionally comprising several subassemblies each adapted to receive and to output an electromotive force, it is frequently the case that when the top threshold is reached, the charge of the battery is distributed in a very unequal manner between its various subassemblies. Hence, one of the subassemblies might thereafter reach by priority its deep discharge threshold and might overdischarge throughout the period required for the discharging of the other subassemblies. Now, a subassembly which remains deeply discharged for too long will experience a great reduction in its lifetime, so that it is a fifth or a sixth thereof for example, this correspondingly shortening the lifetime of the battery.
Furthermore, theoretically, the step of complete charging of each subassembly comprises in particular, in the case of open batteries, a phase of heterogenization of the electrolyte followed by a phase of homogenization: in the course of the first phase of the charging, concentrated electrolyte firstly gradually fills the porous electrodes of the subassembly, then seeps out of these electrodes in the form of a heavy viscous acid which runs down the electrodes and accumulates at the bottom of the bath. This therefore results in a stratification of the electrolyte: the electrolyte concentration becomes high at the bottom of the bath and low at the top of the bath, hence the expression xe2x80x9cheterogenizationxe2x80x9d. During the second phase, the current received by the battery is apportioned into a charging current proper and into an electrolysis current producing oxygen at the positive electrodes and hydrogen at the negative electrodes. The continued charging of the battery therefore causes a release of gas in the bath, thereby causing forced convection of the electrolyte. This results in gradual homogenization of the electrolyte whose concentration ultimately becomes uniform again throughout the height of the bath. The heterogenization phase is sometimes referred to as the xe2x80x9cchargingxe2x80x9d phase, and the homogenization phase as the xe2x80x9coverchargingxe2x80x9d phase. However, at the termination of the heterogenization phase, charging is incomplete and the homogenization phase is merely the continuance of charging so as to obtain complete charging. The use of a top voltage threshold to stop the charging causes the homogenization phase to be shortened and often even to be absent. In this way, a heterogenization (stratification) phase without subsequent complete rehomogenization occurs during each charging period. Consequently, the layers of acid accumulate gradually and irreversibly at the bottom of the bath during the life of the battery. Hence, only the bottom parts of the electrodes participate in the operation of the battery. This leads to their rapid destruction and considerably reduces the lifetime of the electrodes and of the battery.
One sometimes attempts to alleviate these drawbacks by overdimensioning the battery. The effective lifetime of the battery is then longer than that of a more modest battery. Nevertheless, this effective lifetime is substantially reduced relative to that normally envisaged for the battery.
One object of the invention is to provide a method for controlling an electric power plant of the aforesaid type, allowing better knowledge of the effective state of charge of the battery and making it possible to increase the lifetime of the plant without overdimensioning it.
With a view to achieving this object, there is provided according to the invention a method for controlling an electric power plant associated with a power source with temporally random availability, the plant comprising at least one battery adapted to be supplied with electric current from the source, in which method, when the random power is available, the battery current supply is controlled, doing so so as to come as close as possible to a state of complete charge of the battery, and preferably to reach this state.
Thus, the search for the obtaining of the state of complete charge makes it possible to avoid any uncertainty relating to the correlation between the voltage measured across the terminals and the actual state of charge. Once complete charge has been reached, the continuous measurement of the current delivered or received by the battery makes it possible to know at any instant its effective charge with good accuracy.
Moreover, the state of complete charge implies that each of the subassemblies of the battery has also reached its state of complete charge. Subsequently, there is therefore no longer any need to fear premature total discharge on one of the subassemblies, thus making it possible to safeguard the lifetime of the battery.
Furthermore, the complete charging ensures that the phase of heterogenization of the electrolyte has been followed by a phase of sufficient rehomogenization of this electrolyte. This prolongs the lifetime of the electrodes and hence that of the battery.
Finally, the invention avoids the need to resort to an overdimensioning of the plant, which would generate excessive costs.
Advantageously, after the battery has been supplied so as to have received a quantity of electricity above a first predetermined threshold and in particular when the random power is not available, the plant is controlled in such a way that delivery of current by the battery is disabled.
Preferably, the threshold will be below or equal to the partial state of charge after which the electrolyte begins to seep out of the electrodes. Advantageously, this threshold will be as close as possible to this limit. This threshold has a particular value for each type of battery. In certain cases it corresponds to 5% of the nominal capacity of the battery.
Thus, under the aforesaid conditions, the subsequent continuance of the charging phase is favored over the immediate delivery of current. For example, if the phase of heterogenization of the bath has been interrupted for lack of random power, a bid for immediate considerable delivery of current would lead under the effect of the stratification to deep discharging and hence to the fatiguing of the lower parts of the electrodes. It is therefore preferred rather to preserve the battery state for the subsequent resumption of the charging phase causing the continuance of the heterogenization and then the complete rehomogenization of the bath. The lifetime of the battery is thus lengthened.
Advantageously, after the battery has been supplied so as to have received a quantity of electricity below the first threshold and when a delivery of current is requested, the delivery of current by the battery is enabled.
The first threshold is preferably chosen as stated above. Thus, the discharging of the battery is enabled so long as the heterogenization phase has not yet started, namely so long as, at the start of charging, the electrolyte accumulates in the electrodes without seeping out of them. The discharging of the battery does not then cause heterogeneous operation and can be allowed. This involves a limitation to the common control regime stated in the general definition of the invention. This common regime (priority search for the state of complete charge) is preferably limited to the cases where the battery oversteps the first threshold.
This mode of discharge will subsequently be referred to as xe2x80x9cmicrocyclingxe2x80x9d. It is advantageous for several reasons: it makes it possible to reduce the frequency of the complete cycles of the battery. Moreover, the energy yield obtained is high, the concentration of the electrolyte in the porous volume of the electrodes being high.
Advantageously, after the battery has delivered current so as to reach a state of partial nonzero charge, and when the random power is available, the plant is controlled in such a way that current supply to the battery is enabled until it has received a quantity of electricity equal to the first threshold.
Thus, the phase of discharging to the limit of the first threshold is combined with a phase of charging which follows it. Within this limit, it is thus possible to alternate the successive chargings and dischargings so as to make the microcycling last longer.
Advantageously, the plant comprising at least two batteries, in particular which are adapted to be connected in parallel one with respect to the other, after the two batteries or at least two of the batteries have reached a state of partial charge below a second predetermined threshold and when the random power is available, the plant is controlled in such a way that a single first of these batteries is supplied first until it reaches a state of partial charge equal to the second threshold.
Preferably, the second threshold corresponds to the limit between the heterogenization phase and the homogenization phase, or at the very least, is as close as possible to this limit. Its exact value depends on the type of battery. Thus, one favors the supplying of the batteries with current one after the other until the heterogenization phase is completed for the first battery, this phase enabling a larger intensity of current than the homogenization phase which follows it.
Advantageously, after the first battery has reached the second threshold and when the random power is available, the plant is controlled in such a way that the first battery and the or another of the batteries having a state of partial charge below the second threshold are supplied simultaneously.
Thus, after the heterogenization phase has been completed on a battery, the same is done on another battery whilst performing the homogenization phase on the first battery. Thus, the number of batteries in the state of complete charge is optimized when the availability of the random power ceases, for example in the case of solar power, at the end of the day. Hence, after the period of availability, the quantity of stored energy available for the delivery of current is optimized.
Advantageously, when the random power is available, the plant is controlled in such a way that, according to the following order of decreasing priorities:
when the or at least one of the batteries has received a quantity of electricity below the first threshold, the or successfully each of these batteries is supplied until it has received a quantity of electricity equal to the first threshold;
when the or at least one of the batteries has a state of partial charge above the second threshold, the or successively each of these batteries is supplied until it reaches a state of complete charge; and
when the or at least one of the batteries has received a quantity of electricity above the first threshold and has a state of partial charge below the second threshold, the or successively each of these batteries is supplied with current until it reaches a state of partial charge equal to the second threshold.
Thus, the order of decreasing priorities is: microcycling, xe2x80x9coverchargingxe2x80x9d, xe2x80x9cchargingxe2x80x9d (that is to say incomplete charging). This mode of implementation of the method is particularly adapted to cases where it is envisaged that the temporally random power will soon cease to be available for a certain period. For example, when dealing with solar power, this corresponds to a meteorological forecast announcing weak sunshine. In this situation, microcycling is favored vis-à-vis the overcharging and charging phases, the energy stored in the battery or batteries being available immediately.
Advantageously, when the random power is available, the plant is controlled in such a way that, according to the following order of decreasing priorities:
when the or at least one of the batteries has a state of partial charge above the second threshold, the or successively each of these batteries is supplied with current until it reaches a state of complete charge;
when the or at least one of the batteries has received a quantity of electricity below the first threshold, the or successively each of these batteries is supplied with current until it has received a quantity of electricity equal to the first threshold; and
when the or at least one of the batteries has received a quantity of electricity above the first threshold and has a state of partial charge below the second threshold, the or successively each of these batteries is supplied with current until it reaches a state of partial charge equal to the second threshold.
In this mode of implementation, the decreasing priorities are therefore: overcharging, microcycling, charging. This mode is adapted to the cases where it is envisaged that the random power will not be sufficiently available to accomplish the complete charging of at least one battery.
Advantageously, when the random power is available, the plant is controlled in such a way that, according to the following order of decreasing priorities:
when the or at least one of the batteries has a state of partial charge above the second threshold, the or successively each of these batteries is supplied with current until it reaches a state of complete charge;
when the or at least one of the batteries has received a quantity of electricity above the first threshold and has a state of partial charge below the second threshold, the or successively each of these batteries is supplied with current until it reaches a state of partial charge equal to the second threshold; and
when the or at least one of the batteries has received a quantity of electricity below the first threshold, the or successively each of these batteries is supplied with current until it has received a quantity of electricity equal to the first threshold.
Thus, in this mode of implementation, the priorities are: overcharging, charging, microcycling.
It corresponds to envisaged cases other than those stated above. It is therefore adapted to cases where the random power seems to be available for a considerable period of time. For example, for solar power, this is the case of meteorological forecasts indicating strong sunshine during the day.
Advantageously, the plant comprising at least two batteries, in particular which are adapted to be connected in parallel one with respect to the other, when the random power is available and a delivery of current by the plant is requested, the plant is controlled in such a way that current is delivered simultaneously from all the batteries which are not in the charging phase.
Advantageously, after the or one of the batteries has reached a state of zero charge and then goes to the state of complete charge, a quantity of electricity which the battery receives so as to go from the state of zero charge to a state of charge equal to a second predetermined threshold is measured.
Thus, this measurement makes it possible to update the value of the capacity of the battery, this value being liable to vary in the course of the life of the battery. This updating improves the accuracy of the control of the plant, in particular for the tracking of the instantaneous charge of the battery or batteries.
Advantageously, the plant is controlled so as to reach the state of zero charge of the battery.
Thus, the battery is forced to discharge totally with a view to updating the value of its capacity. This operation is performed for example when the total discharging of the battery has not occurred for a predetermined period.
Advantageously, the or at least one of the batteries comprising at least two subassemblies each adapted to receive and to deliver electric current, at least two voltages in series are measured on the battery, each comprising a voltage across the terminals of a respective one of the subassemblies.
Thus, the evolution of the state of charge of each subassembly of the battery is tracked with greater accuracy.
Advantageously, the battery or batteries are furthermore adapted to be supplied with electric current from a power source with temporally definite availability.
The plant may then be referred to as a xe2x80x9chybrid plantxe2x80x9d.
There is also envisaged according to the invention an electric power plant adapted to be associated with a power source with temporally random availability, the plant comprising at least one battery adapted to be supplied with electric current from the source, and means for controlling the plant, in which the control means are adapted to implement the method according to the invention.
Advantageously, the plant comprising at least two batteries, in particular which are adapted to be connected in parallel one with respect to the other, the plant comprises means for disabling the delivery of current by any one of the batteries to the other or to any other of the batteries.
Advantageously, the plant comprises four batteries.
This number makes it possible to control the phases of charging and of discharging of the batteries as a function of the availability of the temporally random power source and of the demand for current, while reconciling good flexibility and reasonable cost.